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Full text of "The Cambridge Natural History"

LIMBS OF SIRENIA

like or rhomboidal to circular in form. Nostrils on upper surface
of not specially -elongated snout. Clavicles are absent. The
scapula has the normal mammalian form, with a well-developed
and roughly median spine. The bones of the arm and hand articu-
late together, as in land animals ; the phalanges show at most
traces of increase in number above the normal. Pelvis represented
by a vestige, more highly developed in some fossil than in recent
forms. Stomach complex, consisting of several chambers. Lungs
simple and not lobnlated. Diaphragm oblique and very muscular.
Brain peculiar in form and but slightly convoluted. Testes ab-
dominal. Teats two, and pectoral in position. Placenta non-
deciduous and zonary.1

This limited group consists of purely aquatic forms^ which are
both marine and fresh-water in their proclivities. They have
been placed in the immediate vicinity of the Whales ; but it is
now believed by most zoologists that the likenesses which they
undoubtedly show to the Cetacea are of an adaptive kind and
related to their similar mode of life. The group is a readily-
definable one. Externally they are marked by their dark
coloration, somewhat Whale-like though of clumsier build, and by
the total absence of external ears and hind-limbs ; the latter are,
however, as will be pointed out shortly, marked by certain rudi-
mentary bones. There is a flattened tail, which in the Dugong and
JKhytina is precisely like that of a Whale. It is interesting to
note that the former genus, whose tail is, judging it at least by
the standard of the Whales, more completely modified for the
aquatic life, should also show other features which indicate their
longer life as marine creatures. , For the flippers are more Whale-
like in that the fore-arm is completely enclosed within the body,
or nearly so, and the nostrils have a more decidedly superior
position than in the Manatee. The fore-limbs of this group, as
may be inferred from what has just been said, are flipper-like ;
but, contrary to what we find in Whales, the phalanges do not as
a rule show any traces of multiplication, so characteristic a feature
of the Cetacean hand, and the individual bones are connected by
well-formed joints. Beneath the thick skin, which is sparsely
provided with stout hairs in the Dugong, is a layer of blubber.
Dr. Murie has called attention to the fact that this layer in the

probably also in Jtffwa&us.   See Turner, Trans. Moy,